The present invention generally relates to a sheath intended to cover, for protective purposes, a flexible tube or hose of the kind used for conveying a working fluid in a hydraulic or pneumatic circuit, in addition to flexible tubes of other kinds.
Use of protective sheaths to cover flexible tubes or hoses conveying a working fluid, by which hydraulic or pneumatic devices of a piece of equipment are actuated, is known in the art. Generally, the sheaths consist of a rather flexible strip of plastic material that is helically wound around an outer surface of the flexible tubes. A main object of the sheaths is to protect the flexible tubes from wear consequent to continued action of friction, for example, when the movable members of a piece of equipment rub against the flexible tubes during movement relative thereto.
Because pieces of equipment, whereon hydraulic or pneumatic devices are installed, frequently operate in extreme working conditions with regard to mechanical and thermal stresses, protective sheaths must provide, apart from a good resistance to abrasion, also a good resistance to flame, i.e. plastic material from which a sheath is made from should be, as much as possible, a flame retardant material. Another feature the protective sheaths should have is to be electrically conductive so as to permit the electrostatic charges, which form locally on surface portions of a flexible tube because of a frictional electrification phenomenon when a working fluid having insulating properties is passed through the tube, to be dissipated to earth. These electrostatic charges, if not suitably dissipated, may accumulate in such an amount that they generate a high electric potential gradient with respect to earth and produce dangerous electric discharges through working ambient air. For this reason, material from which the protective sheath is made should have an electrical resistance per unit length that is not greater than a fixed value which is established by International Standards so as to effectively dissipate the electrostatic charges.
For a purpose of making sheaths electrically conductive, the sheaths comprise a portion of electrically conductive material, for example in the form of a conductive wire or strip which follows a helical path of a sheath along an entire length thereof and is kept in contact with an outer surface of a flexible tube when the sheath is wound thereon.
However, sheaths of the above-mentioned kind suffer from some disadvantages among which include: high cost of an electrically conductive wire or strip; reduced flexibility of a sheath due to presence of the electrically conductive wire or strip; and the high manufacturing cost of the sheath.
Another disadvantage of a protective sheath of the above-mentioned kind relates to the fact that with a conventional manufacturing process a portion of the electrically conductive wire or strip intended to contact the outer surface of the tube may be covered by insulating plastic material of the sheath, whereby the insulating plastic material is interposed between the outer surface of the flexible tube and the electrically conductive wire or strip and negatively affects capability of the sheath to dissipate electrostatic charges locally formed on outer surface portions of the flexible tube.
An object of the present invention is to provide a protective sheath with improved features with regards to its ability to resist abrasion and flame and to be electrically conductive, and which sheath is adapted to assure during operation a degree of safety and reliability greater than that obtainable with prior art protective sheaths which are available on the market.
According to the present invention, a protective sheath intended to cover a flexible tube, used for conveyance of a working fluid to a hydraulic or pneumatic device, is formed of a strip which is helically wound around an outer surface of the flexible tube and is characterized in that the strip comprises:
a core of electrically insulating and flame retardant material; and
a core-covering layer of electrically conductive and flame retardant material having a first surface portion intended to contact the outer surface of the flexible tube, and a second surface portion opposite to the first surface portion and forming an on sight surface of the strip when it is wound around the flexible tube,
with the core-covering layer of electrically conductive material being intended to dissipate electrostatic charges which are formed on the outer surface of the flexible tube, and to protect the outer surface of the flexible tube from wear and flame by performing a flame retardant action.